Before an organic or inorganic coating film is overcoated with another coating film, the adhesiveness of the surface of the coating film must be improved. Various techniques have been utilized for this purpose, including roughening of the surface using sandpaper or by a honing treatment, along with activating the surface by a plasma or corona discharge treatment or by irradiation with ultraviolet light.
Among many kinds of coating compositions used in the filed of electronic technology, silicone-based coating compositions are widely used due to their excellent electrical insulation, heat resistance and stability characteristics. In the manufacture of semiconductor devices, such as VLSIs, for example, silicone-based coating compositions are used as an inter-layer insulation film in the process of multi-layer circuit formation (Japanese patent publication No. 60-46826). Additionally, in the manufacture of liquid crystal display units, such coatings are used to form orientation films (Japanese patent publication No. 60-25851).
When a coating film of a silicone-based coating composition is to be overcoated with another coating film, such as a film of photoresist or a film formed by a CVD or PVD process, good adhesion between the coating films or an improvement of the adhesive bonding strength is desired. In the adhesive bonding during the assembly of liquid crystal display units, the most effective method for obtaining suitable adhesion is to subject the surface of the silicone-based coating film to a treatment with oxygen plasma so as to convert the silicone to silicon oxide. This method, however, is not quite satisfactory from a practical standpoint because cracks are sometimes formed in the coating film.
In Japanese patent Kokai No. 57-111046, a solution has been proposed to this problem, in which the coating film comprises a specific kind of silicone-based coating materials, and a polysilsesquioxane is provided with an overcoating layer of silica (SiO.sub.2) or alumina (Al.sub.2 O.sub.3) in order to improve the adhesiveness of the surface without producing such cracks. This method, however, is also not completely satisfactory because of the relatively high costs involved as a consequence of the additional equipment needed for forming the additional film on the silicone-based coating.
Accordingly, it is desired to develop a method for uniformly improving the surface of a silicone-based coating film without causing crack formation in or exfoliation (i.e., lifting off) of the coating film.
In the manufacture of various kinds of electronic components utilizing such a silicone-based coating film, patterning of the silicone-based coating film on the substrate is usually necessary. In this process, the surface of a silicone-based coating film is coated with a photoresist composition to form a photoresist layer which is then subjected to patterning by conventional photolithographic techniques. Thereafter, the bare, exposed areas of silicone-based coating film are subjected to etching by the RIE method, with the patterned photoresist layer serving as a mask, and followed by the removal of the photoresist layer by a plasma gas treatment using oxygen or a gas mixture mainly composed of oxygen.
This method, however, has a serious problem in that cracks are sometimes formed in the silicone-based coating film during plasma treatment so that it is very difficult to obtain a patterned silicone-based coating film of high quality. This problem can be partly solved by providing an intermediate layer of silica (SiO.sub.2) or alumina (Al.sub.2 O.sub.3) on the coating film, as described in Japanese patent publication No. 57-111046. Such an intermediate layer is effective to relax stress in the film during plasma treatment so that formation of cracks in the silicone-based coating film can be prevented. This method is of course not quite satisfactory from the practical standpoint due to the high equipment and processing costs for the additional coating. Accordingly, it is desired to develop an improved method for patterning a silicone-based coating film without forming cracks during the manufacture of various kinds of electronic components.